1. Field of the Invention
This invention relates to cordless telephone system having a speaker phone function.
2. Prior Art
Cordless telephone systems are generally constituted by a base station directly connected to the telephone line, and handset units which are wireless and portable and associated with the base station through two-way radio communication. Some of such cordless telephone systems provide the speaker phone function allowing users to enjoy conversation without using their hands. The speaker phone function is bidirectional in enabling the users to have a conversation with an opponent party without using their hands in both telephone transmission and reception. Thus, the speaker phone function is convenient when compared with a unidirectional on-hook dial function which only allows the users to rest their hands in a telephone reception mode.
A fundamental operation of conventional cordless telephone system, comprising a base station and an associated handset unit both having the speaker phone function, will be explained below.
When a user depresses a speaker phone button on the base station, a control unit of the base station operates its own speaker phone circuit to output audio signals received from the handset unit or the telephone line through a speaker not through the handset. On the contrary, in response to the user's voice, the base station causes its transmitting section to convert the received voice into audio signals and transmit them to the opponent party.
Meanwhile, when a user of the handset unit depresses a speaker phone button on the handset unit, the handset unit causes its control section to operate its own speaker phone circuit in the same manner as the base station.
However, according to the above-described conventional cordless telephone system, the speaker phone circuit must be installed in each of the base station and the handset unit. The speaker phone circuit is generally constructed by an analog IC (integrated circuit), whose size is fairly large, for example, 14.times.36 mm for 28 pins. Provision of such a large IC is not desirable for the handset unit, since the handset unit needs to reduce the weight and size to provide handiness in use. Furthermore, cost of such an analog IC is too expensive to provide it in each of the base station and handset units.